Special requirements are made with regard to dosage inhalators intended for local administration of drugs to the respiratory tract and to the lungs. Since mostly very potent drugs are to be administered, the dose accuracy must be great. The dosage of active compound that is to be administered may be as small as 0.1 mg. It is also necessary that the particles that leave the dosage inhalator have a suitable size distribution, since too big particles tend to be deposited in the mouth. A typical formulation of isoproteranol sulfate powder intended for administration as an aerosol is described in the U.S. Pharmacopeia of July 1, 1980 as being sufficiently powdered that the great majority of individual particles are less than 5 .mu.m in diameter. The British Pharmaceutical Codex of 1973 similarly discloses that in both ergotamine sulfate aerosols and isoprenaline sulfate aerosols most of the individual particles have a diameter of not more than 5 .mu.m.
Several systems are available for local administration of drugs to the respiratory tract and to the lungs. Among these systems may be mentioned nebulizing devices, pressurized aerosols, pump inhalators, and inhalators which are activated by the air flow generated at inhalation, herebelow denoted "powder inhalators".
Several types of powder inhalators are available on the market. They represent a complement to pressurized aerosols, where the drug is dissolved or suspended in a liquid propellant mixture. The powder inhalators have the advantage that they always deliver the active compound when the patient inhales, as the particle cloud is generated by the air flow obtained at inhalation. Thereby the problem of coordinating the activation of a dosage with the inhalation in order to bring the active compound to the respiratory tract and to the lungs is solved. An example of powder inhalators which are available on the market is Spinhaler.RTM.. In Spinhaler.RTM. the active compound is used in micronized form, contained in a hard gelatine capsule which is perforated before use. The hard gelatine capsule is placed in a tube in a device which is brought to rotation by the air flow generated at inhalation, whereby the micronized compound is moved into the air stream and is brought via the air flow to the respiratory tract and lungs of the patient.
However, the previously known powder inhalators have disadvantages:
(1) they cannot be used for accurate and reproducible dispensing of micronized active compound in amounts below 15-20 mg, and they are therefore useful only for less potent active compounds or for highly potent active compounds in combination with a diluting agent, usually lactose;
(2) they are cumbersome to load and to make clean;
(3) usually several inhalations are required to empty a capsule containing a unit dose;
(4) they are difficult to handle for patients with reduced breathing capacity or with reduced capability in the use of their hands and;
(5) lactose, the diluting agent, is very disturbing at inhalation and may increase the frequency of caries.
There is a need for an effective powder inhalator which is activated by the air flow generated at inhalation, which is easy to handle for the patient, which allows dispensation of active compound in an amount down to 0.1 mg without need to include any diluting agent, and which gives a suitable size distribution for the particles which are administered.